What soils are reactive and where are they found?

Essentially, reactive soils are any soil that undergoes a major change in volume at different moisture levels. Due to their extremely small grain size, which can be penetrated by water, clays are typically the most reactive soil substrate. Clay is obviously a very general term as it provides little description of the soil composition, however, smectite, illite and vermiculite have the greatest water absorbing qualities of common clays (Sheard and Bowman, 1996). Unsurprisingly, the majority of Adelaide's clay soils are comprised of the aforementioned minerals, making Adelaide highly susceptible to soil reactivity. In Adelaide, the areas most heavily affected by soil reactivity are the Western and North-western suburbs, particularly around the Hindmarsh and Keswick areas. While these are the two areas with the most soil movement, there are two predominant soil types; one known as Hindmarsh Clay and the other Keswick Clay, named obviously for their places of origin. According to the Building Services Authority of Queensland, highly reactive clays can move in excess of 75mm in an annual cycle (BSA, 2011). While this number is extreme, the Adelaide climate combined with Hindmarsh and Keswick clays can provide ground movement somewhat similar to this figure. Such extremes of subsoil movement take their toll on buildings, which are designed to millimetre perfection without the intention of movement. Inducing movement on a precise structure can easily destabilise its structure and fracture its foundations, resulting in catastrophic outcomes for residents.

While other areas of Adelaide do not have soils as reactive as Keswick and Hindmarsh, the climate in Adelaide gives potential for even less reactive soils to cause damage to footheld structures. Surrounding the city, in all directions, the soils are reasonably reactive as Keswick and Hindmarsh clays are commonplace. Heading further west and southwest from the city, the soil becomes more sandstone heavy and hence is less reactive. In a northerly direction, the soil becomes more quartz-rich and again less reactive. So in general, soils are more reactive in the CBD and surrounding suburbs, getting progressively less reactive the further away from the city one moves (Sheard and Bowman, 1996). Figure 1 (Below) Shows areas where the predominant soil type is that of Keswick Clay, while the naming of the soil would imply otherwise, it is not simply limited to the Keswick region. It can be seen that the majority of the highly reactive Keswick Clay is centered around the city, extending primarily in a north to north-easterly direction. Areas with high Keswick Clay content, are typically the areas worst affected by soil reactivity.

Fig 1: A topographical map of the Adelaide region, areas highlighted are known to have Keswick Clay present, through borehole drilling and testing.